Polarized Sunglasses

Polarized Sunglasses If you rotate a pair of polarizing sunglasses, you will find that they cut road glare much better in some positions than in others.

When light reflects from water, asphalt, or other non-metallic surfaces, it becomes polarized. That is, the reflected light is usually vibrating more in one direction than in others. Polarizing sunglasses reduce this reflection, known as glare, but only when the polarizing lenses are oriented properly.

One or two pieces of polarizing material (such as old lenses from polarizing sunglasses).

One clear light bulb with socket and cord.

One piece of shiny opaque plastic (the shiny black side of a cassette tape case will work well).

No assembly necessary. (15 minutes or more)

Place the lit bulb with its filament parallel to the surface of the plastic. Orient the bulb so that you can see the reflection of the bulb in the plastic.

Look at the reflection through a piece of polarizer. Rotate the polarizer and vary the angle at which you look at the plastic until you get the dimmest reflection. You'll probably get the best results when there's about a 35-degree angle between your eyes and the piece of plastic (see drawing). Rotate the polarizer 90 degrees as you watch the reflection. The reflection should become notably brighter.

Observe reflections elsewhere around you. Rotate the polarizer and vary the angle of viewing to vary the brightness. Try looking at a reflection from a metallic surface, such as an ordinary mirror. There should be no difference in the brightness of an image reflected in the mirror as you rotate the polarizer or vary the angle of viewing.

Look at the sky through the polarizing lens. Notice that the brightness of the sky changes as you rotate the polarizer. That's because the light in the sky is polarized. (For more information about this effect, see the Blue Sky Snack.)

Look through a polarizer at the surface of a pond on a bright, sunny day. Rotate the polarizer and notice that at one orientation of the polarizer, the surface reflections are greatly reduced and you can see beneath the surface of the water. Rotate the polarizer 90 degrees from this orientation, and the surface reflections block your view of the underwater world. This is why fishermen wear polarizing sunglasses.

The light bulb produces unpolarized light - each photon is vibrating in its own different direction. Nonmetallic surfaces, such as black plastic, tend to reflect light that is vibrating parallel to the surface and to transmit or absorb light vibrating in all other directions. If the black plastic is horizontal, then it reflects light that is vibrating horizontally, creating horizontally polarized light. The horizontal black plastic reflects less light that is vibrating vertically.

The polarizer lets through light that is vibrating in one direction and absorbs light that is vibrating in all other directions. When the black surface is horizontal, the reflection looks dimmest when you hold the filter so that it lets through just vertically vibrating light. The reflection looks brightest when you hold the filter so that it lets through just horizontally vibrating light. Horizontal surfaces in the environment, such as the asphalt of a street or the surface of a lake, reflect light that is vibrating horizontally. Polarizing sunglasses absorb this horizontally oriented glare. If you tilt your head sideways, this horizontally oriented glare passes through the glasses, making the surface look brighter.

Light becomes completely polarized parallel to the surface at one particular angle of reflection, called Brewster's angle. Brewster's angle for water is 53 degrees; for glass it is 56 degrees; for plastic the angle varies but, in general, will be somewhere between these two angles. Brewster's angle is traditionally measured from a line that is perpendicular to the surface. To find the angle measured directly from the surface you must subtract Brewster's angle from 90 degrees.